linux-kernel - Re: [PATCH Part2 RFC v2 32/37] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT

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Date:   Mon, 10 May 2021 15:50:57 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm list <kvm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, jroedel@...e.de,
        "Lendacky, Thomas" <thomas.lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>, peterz@...radead.org,
        "H. Peter Anvin" <hpa@...or.com>, tony.luck@...el.com
Subject: Re: [PATCH Part2 RFC v2 32/37] KVM: SVM: Add support to handle MSR
 based Page State Change VMGEXIT


On 5/10/21 2:59 PM, Peter Gonda wrote:
> On Mon, May 10, 2021 at 11:51 AM Brijesh Singh <brijesh.singh@....com> wrote:
>> Hi Peter,
>>
>> On 5/10/21 12:30 PM, Peter Gonda wrote:
>>>> +static int snp_make_page_shared(struct kvm_vcpu *vcpu, gpa_t gpa, kvm_pfn_t pfn, int level)
>>>> +{
>>>> +       struct rmpupdate val;
>>>> +       int rc, rmp_level;
>>>> +       struct rmpentry *e;
>>>> +
>>>> +       e = snp_lookup_page_in_rmptable(pfn_to_page(pfn), &rmp_level);
>>>> +       if (!e)
>>>> +               return -EINVAL;
>>>> +
>>>> +       if (!rmpentry_assigned(e))
>>>> +               return 0;
>>>> +
>>>> +       /* Log if the entry is validated */
>>>> +       if (rmpentry_validated(e))
>>>> +               pr_debug_ratelimited("Remove RMP entry for a validated gpa 0x%llx\n", gpa);
>>>> +
>>>> +       /*
>>>> +        * Is the page part of an existing 2M RMP entry ? Split the 2MB into multiple
>>>> +        * of 4K-page before making the memory shared.
>>>> +        */
>>>> +       if ((level == PG_LEVEL_4K) && (rmp_level == PG_LEVEL_2M)) {
>>>> +               rc = snp_rmptable_psmash(vcpu, pfn);
>>>> +               if (rc)
>>>> +                       return rc;
>>>> +       }
>>>> +
>>>> +       memset(&val, 0, sizeof(val));
>>>> +       val.pagesize = X86_TO_RMP_PG_LEVEL(level);
>>> This is slightly different from Rev 2.00 of the GHCB spec. This
>>> defaults to 2MB page sizes, when the spec says the only valid settings
>>> for level are 0 -> 4k pages or 1 -> 2MB pages. Should this enforce the
>>> same strictness as the spec?
>>
>> The caller of the snp_make_page_shared() must pass the x86 page level.
>> We should reach here after all the guest provide value have passed
>> through checks.
>>
>> The call sequence in this case should be:
>>
>> snp_handle_vmgexit_msr_protocol()
>>
>>  __snp_handle_page_state_change(vcpu, gfn_to_gpa(gfn), PG_LEVEL_4K)
>>
>>   snp_make_page_shared(..., level)
>>
>> Am I missing something  ?
> Thanks Brijesh. I think my comment was misplaced. Looking at 33/37
>
> +static unsigned long snp_handle_page_state_change(struct vcpu_svm
> *svm, struct ghcb *ghcb)
> +{
> ...
> + while (info->header.cur_entry <= info->header.end_entry) {
> + entry = &info->entry[info->header.cur_entry];
> + gpa = gfn_to_gpa(entry->gfn);
> + level = RMP_TO_X86_PG_LEVEL(entry->pagesize);
> + op = entry->operation;
>
> This call to RMP_TO_X86_PG_LEVEL is not as strict as the spec. Is that OK?

I am not able to follow which part of the spec we are missing here. Can
you please elaborate it a bit more - thanks

The entry->pagesize is boolean, so, the level returned by the macro is
either a 4K or 2MB.